In a typical Q-switched laser system containing an intracavity nonlinear optical frequency converter, the strong fields generated within the resonator permit much higher single-pass conversion efficiencies than can be obtained by placing the crystal outside the resonator. For a low repetition rate, high power Q-switched laser, intracavity frequency conversion offers little real advantage over external conversion. However, in high repetition rate, high average power Q-switched systems (such as continuously pumped, acousto-optically Q-switched lasers), intracavity conversion represents a significant advantage. To date, any nonlinear optical crystal used for frequency conversion in such devices is a passive component, in the sense that the single-pass efficiency with which the crystal converts the frequency of the fundamental beam(s) to a harmonic frequency is determined by the laser's average and peak power levels. Small variations in the efficiency of the conversion process can be realized by adjusting the crystal orientation and temperature.
This passively coupled scheme has several disadvantages. During the time in which the laser field is building up from a noise floor to the laser oscillation threshold, the crystal is continuously converting a small fraction of the intracavity optical energy from its fundamental frequency to a higher harmonic frequency, and such energy conversion represents a loss that increases as the energy of the fundamental frequency increases. The loss reduces the peak efficiency of the conversion process, because it reduces the peak energy in the resonator.
Small fluctuations in both the energy stored in the laser medium prior to the opening of the Q-switch and the amplitude and phase of the noise level from which the oscillation begins cause greatly magnified variations in the time at which the laser system emits a coherent light pulse at the higher harmonic frequency. This "jitter" can cause significant timing problems in detection schemes that rely on the stability of the laser signal. Accordingly, it will be evident that precisely controlling the time at which a Q-switched laser emits a pulse of coherent light can substantially eliminate problems with jitter.
Nonlinear optical crystals are also used to mix the frequencies of two lasers that share a common portion of an internal optical path. When the frequencies of two lasers are mixed in a single nonlinear optical crystal, the sum-frequency mixing efficiency is optimal only if the two lasers reach threshold at precisely the same time. In practice, this task is difficult to achieve, even for two uncoupled lasers, but it becomes particularly so in a coupled system, because intense electrical fields in one oscillating laser rob energy from the fields in the second. Controlling the timing at which two lasers in a coupled system reach threshold is closely related to the problem of controlling jitter in the output of a single Q-switched laser. In both instances, the key to solving the problem lies in controlling the function performed by the nonlinear optical crystal.